The following discussion and the description of the invention will be set forth primarily for aluminum cans, however, both the discussion and the description of the invention apply also to tin plated steel cans and to other types of formed metal surfaces for which any of the above stated intended purposes of the invention are of interest.
Aluminum cans are commonly used as containers for a wide variety of products. After their manufacture, the aluminum cans are typically washed with acidic or alkaline cleaners to remove aluminum fines and other contaminants therefrom. Treatment of aluminum cans with either alkaline or acidic cleaners generally results in differential rates of metal surface etch on the outside versus on the inside of the cans. For example, optimum conditions required to attain an aluminum fine-free surface on the inside of the cans usually leads to can mobility problems on conveyors because of the increased roughness on the outside can surface. Aluminum cans that lack a low coefficient of static friction (hereinafter often abbreviated as “COF”) on the outside surface usually do not move past each other and through the trackwork of a can plant smoothly. Clearing the jams resulting from failures of smooth flow is inconvenient for the persons operating the plant and costly because of lost production.
The COF of the internal surface is also important when the cans are processed through most conventional can decorators. The operation of these machines requires cans to slide onto a rotating mandrel which is then used to transfer the can past rotating cylinders which transfer decorative inks to the exterior surface of the cans. A can that does not slide easily on or off the mandrel cannot be decorated properly and results in a production fault called a “printer trip”. In addition to the misloaded can that directly causes such a printer trip, three to four cans before and after the misloaded one are generally lost as a consequence of the mechanics of the printer and conveyor systems.
There is a need in the can manufacturing industry, particularly with aluminum cans, to modify the COF on the outside and inside surfaces of the cans to improve their mobility. Generally, the COF is reduced by the use of an aqueous surface treatment that includes a mobility enhancer. An important consideration in modifying the surface properties of cans is the concern that such modification may interfere with or adversely affect the ability of the cans to be printed when passed to a printing or labeling station. For example, after cleaning the cans, labels may be printed on their outside surface, and lacquers may be sprayed on their inside surface. In such a case, the adhesion of the paints, labels and lacquers is of major concern. It is therefore an object of this invention to improve mobility without adversely affecting adhesion of paints, decorating inks, lacquers, or the like. Another cause of printing and labeling defects is the presence of visible waterbreaks on the can surfaces. It is desirable that the amount of waterbreak on the cans be minimized. However, often the very component that enhances mobility of the can, e.g. oil or a particular surfactant, will increase the amount of waterbreak seen on the can surfaces.
In addition, the current trend in the can manufacturing industry is directed toward using thinner gauges of aluminum metal stock. The down-gauging of aluminum can metal stock has caused a production problem in that, after washing, the cans require a lower drying oven temperature in order to pass the column strength pressure quality control test. However, lowering the drying oven temperature resulted in the cans not being dry enough when they reached the printing station, which in turn caused label ink smears and a higher rate of can rejects. One solution to the problem of insufficient drying in the lower temperature drying oven is allow the cans to bake for longer, but this is economically impractical. A better solution is to reduce the amount of water remaining on the surface of the cans that is carried into the drying oven. Thus, it would be advantageous to have a lubricant and surface conditioner composition that promotes the drainage of rinse water from the treated can surfaces.
In summary, it is desirable to provide a means of improving the mobility of aluminum cans through single filers and printers to increase production, reduce line jams, minimize down time, reduce can spoilage, improve or at least not adversely affect ink laydown, and enable lowering the drying oven temperature of washed cans. Past improvements in this respect have led to increases in conventional can processing speeds, so that only the lower part of the range of previously acceptable COF values is now acceptable in many plants. One such improvement is disclosed in U.S. Pat. No. 6,040,280, the entire specification of which, except to any extent that it may be inconsistent with any explicit statement herein, is hereby incorporated herein by reference. The invention taught in the '280 patent provided good mobility, i.e. lowered the COF and slip angle, of cans treated therewith. One drawback of the '280 patent is the limited availability of raw materials required to make the mobility enhancer. Also, there is still a need to provide improvements over the '280 patent teachings such as a composition which can provide improvements in at least one of mobility performance, uniform wetting (low % waterbreak), drainage and bake-off characteristics. It is particularly desirable to provide a surface conditioner that decreases the amount of water carried on cans into the drying oven and that resists baking off in the oven.
In the most widely used current commercial practice, at least for large scale operations, aluminum cans are typically subjected to a succession of six cleaning and rinsing operations as described in Table A below. It is preferable to include another stage, usually called “Prerinse”, before any of the stages shown in Table A; when used, this stage is usually at ambient temperature (i.e., 20-25 degrees C.) and is most preferably supplied with overflow from Stage 3 as shown in Table A, next most preferably supplied with overflow from Stage 1 as shown in Table A, and may also be tap water. Any of the rinsing operations shown as numbered stages in Table 1 may consist of two or preferably three sub-stages, which in consecutive order of their use are usually named “drag-out”, “recirculating”, and “exit” or “fresh water” sub-stages; if only two sub-stages are used, the name “drag-out” is omitted. Most preferably, when such sub-stages are used, a blow-off follows each stage, but in practice such blow-offs are often omitted. Also, any of the stages numbered 1 and 4-6 in Table A may be omitted in certain operations.
TABLE AStage NumberAction On Surface During Stage1Aqueous Acid Precleaning2Aqueous Acid and Surfactant Cleaning3Tap Water Rinse4Mild Acid Postcleaning, Conversion Coating,or Tap Water Rinse5Tap Water Rinse6Deionized (“DI”) Water Rinse
An object of the present invention is to provide a lubricant and surface conditioner forming composition that will achieve satisfactory COF reduction, as shown by reduced slip angles, when used as the last aqueous treatment before drying the cans (“final rinse”), even on can surfaces already coated with a conversion coating by an earlier treatment stage. An alternative and/or concurrent objective is to overcome at least one of the difficulties with the prior art noted above. Other objects will be apparent from the further description below.